Barium lanthana borosilicate glasses are well known for optical glass purposes as illustrated for example by U.S. Pat. No. 2,297,453 granted Sept. 29, 1942 to Berger and Rehm, No. 2,518,028 granted Aug. 8, 1950 to N. J. Kreidl, No. 2,523,362 granted Sept. 26, 1950 to W. A. Fraser et al., No. 2,576,521 granted Nov. 27, 1951 to Kreidl and Brewster, No. 3,081,178 granted Mar. 12, 1963 to G. Weissenberg et al., and No. 3,248,238 granted Apr. 26, 1966 to M. Faulstich. In general, the prior art, exemplified by these patents, has recognized that, while increasing the lanthana content of a glass is highly beneficial to the refractive index of a glass, it also creates a crystallization problem. To counter this, lanthana is either limited below ten percent, or relatively large contents of BaO and/or B.sub.2 O.sub.3 are recommended. The latter also is used to attain a high dispersive index.
While these measures have mitigated, to some extent at least, the optical and devitrification problems, they have also resulted in diminished chemical durability. This can be tolerated in some optical glass applications, particularly where individual finishing is practiced. However, it leads to a serious glass staining problem in automated grinding and polishing operations now utilized to finish optical glass lenses and the like.
The prior art has recommended such additives as zirconia, titania, thoria and alumina to improve chemical durability, but Kreidl in U.S. Pat. No. 2,518,028 recognized that lanthanum barium borosilicates containing these additives were still chemically unstable and tended to dissolve in acidic media in general. Accordingly, he minimized lanthana and proposed adding such oxides as tantala (Ta.sub.2 O.sub.5), niobia (Nb.sub.2 O.sub.5) also known as columbium oxide, and cadmium oxide (CdO). However, the scarcity and high cost of such additives make this solution to the problem unsatisfactory.